1. Field of the Invention
This invention relates to an apparatus for measuring an air flow intake of an internal combustion engine, and more particularly is directed to an apparatus including an air flow sensor for measuring the level of the air flow intake of an internal combustion engine and an interface circuit for receiving an output signal from this sensor.
2. Description of the Prior Art
As is widely known, an air flow sensor for measuring the air flow intake of an internal combustion engine may include a hot wire type air flow sensor. The hot wire type air flow sensor acts to output a voltage signal corresponding to the amount of air passing through the sensor. An input processing circuit receives the voltage signal output from the sensor and such a processing circuit subsequently processes and transmits the received signal to an analog-to-digital conversion IC. A fuel injection control unit incorporating such an input processing circuit judges the amount of air flow sucked in by the internal combustion engine and sprays an amount of fuel corresponding to the amount of air flow.
FIG. 1 illustrates a conventional hot wire type air flow sensor 1, and FIG. 2 shows a combination of the air flow sensor 1 shown in FIG. 1 and a fuel injection unit 2 including an input processing circuit 2a according to the prior art.
In FIG. 1, symbol R.sub.H designates a hot wire (heating resistor), R.sub.K a thermistor for sensing the temperature of the air flow in an internal combustion engine, R.sub.A and R.sub.B reference resistors, 1a an amplifier, 1d a differential amplifier and 1e a transistor. In the conventional air flow sensor, a current I.sub.H flows in the hot wire R.sub.H under the control of a feed back loop comprising the differential amplifier 1d and the transistor 1e, and corresponds to the level of the air flow to keep the temperature of the hot wire R.sub.H constant. Therefore, it is possible to detect the level of air flow intake from the current I.sub.H, and an output voltage V.sub.AFS (=A.multidot.I.sub.H .multidot.R.sub.B ; A: a gain of the amplifier 1a) corresponds to the level of air flow. The fundamental constitution and operation of such a conventional air flow sensor are described in Japanese Patent Public Disclosure No. 76182/79.
Turning to FIG. 2, the input processing circuit 2a in the fuel injection unit 2 is connected to the air flow sensor 1 which is constructed as shown in FIG. 1 and includes an operational amplifier 2a.sub.1. The output voltage V.sub.AFS from the amplifier 1a is applied to a non-inverting input terminal (positive input terminal) of the operational amplifier 2a.sub.1.
This amplifier 2a.sub.1 is adapted to amplify an input voltage V.sub.IN and to generate an output voltage V.sub.o to an analog to digital converter (converter) not shown in FIG. 2. An inverting input terminal (negative input terminal) of the amplifier 2a.sub.1 is grounded via a resistor R.sub.12, and a resistor R.sub.11 is connected between an output terminal and the negative input terminal thereof.
The input processing circuit 2a functions to process the output voltage V.sub.AFS from the amplifier 1a and transmits the processed voltage to the A/D converter. The processed, or output voltage V.sub.o of the circuit 2a can be represented by the following equation: EQU V.sub.o =V.sub.IN .times.(R.sub.11 +R.sub.12)/R.sub.12
Note that numeral 3 shown in these drawings designates a battery for supplying electrical power to the fuel injection unit 2 as well as to the air flow sensor 1.
In the apparatus shown in FIG. 2, the amplifier 1a incorporated in the air flow sensor outputs the voltage V.sub.AFS on the basis of the potential at a reference point A.sub.1 in the sensor 1. The potential at the reference point A.sub.1 is a positive potential V.sub.1 but not a ground potential at the earth point E.sub.1 when the sensor 1 is energized. That is because a line l.sub.1 connected between the points A.sub.1 and E.sub.1 has a resistance, even though the value thereof is small, whereby a voltage drop is caused on the line l.sub.1 when a current I.sub.1 flows from the sensor to the ground point E.sub.1 through the line l.sub.1.
Similarly, in the fuel injection unit 2, a reference point A.sub.2 has a positive potential V.sub.2 on the basis of a ground point E.sub.2, which is equal to the voltage drop between the reference point A.sub.2 and the ground point E.sub.2 and is caused by a resistance of a line l.sub.2 connected between the points A.sub.2 and E.sub.2 and a current I.sub.2 therethrough.
Furthermore, since the current I.sub.2 is run to the battery 3, a potential difference V.sub.12 is caused between the ground points E.sub.1 and E.sub.2.
Hence, the voltage V.sub.IN input to the input processing circuit 2a, which is based on the reference point A.sub.2, is expressed such as: EQU V.sub.IN =V.sub.AFS -(V.sub.2 +V.sub.12 -V.sub.1) EQU .BECAUSE.V.sub.AFS -V.sub.1 =V.sub.IN -(V.sub.2 +V.sub.12)
It follows from this formula that an input error represented by (V.sub.2 +V.sub.12 -V.sub.1) is given to the operational amplifier 2a.sub.1. Accordingly, the output voltage V.sub.o includes the above error and the error is also transmitted to the A/D converter, thereby the fuel injection unit 2 comes to have an error in fuel injection quantity, which responds to the input error (V.sub.2 +V.sub.12 -V.sub.1).